Declarative memory, also referred to as explicit memory is one of the two main types of long-term human memory. It is the conscious, intentional recollection of factual information, previous experiences and concepts. Declarative memory can be divided into two categories: episodic memory, which stores specific personal experiences, and semantic memory, which stores factual information.
Declarative memory’s counterpart is known as implicit memory or procedural memory, which refers to unconscious memories such as skills (e.g. knowing how to get dressed, eat, drive, ride a bicycle without having to re-learn the skill each time).
People use explicit memory throughout the day, such as remembering the time of an appointment or recollecting an event from years ago. Explicit memory involves conscious recollection, compared with implicit memory which is an unconscious, unintentional form of memory. Remembering a specific driving lesson is an example of explicit memory, while improved driving skill as a result of the lesson is an example of implicit memory.
Sometimes, the distinction between explicit memory and declarative memory is made. In such cases, explicit memory relates to any kind of conscious memory, and declarative memory relates to any kind of memory that can be described in words; however, if we assume that a memory cannot be described without being conscious and vice versa, then the two concepts are identical.
Types Of Declarative Memory
Episodic memory consists of the storage and recollection of observational information attached to specific life-events. These can be memories that happened to the subject directly or just memories of events that happened around them. Episodic memory allows for mental time travel – recalling various contextual and situational details of one’s previous experiences.
Some examples of episodic memory include the memory of entering a specific classroom for the first time, the memory of storing your carry-on baggage while boarding a plane headed to a specific destination on a specific day and time, the memory of being notified that you are being terminated from your job, or the memory of notifying a subordinate that they are being terminated from their job. The retrieval of these episodic memories can be thought of as the action of mentally reliving in detail the past events that they concern. Episodic memory is believed to be the system that provides the basic support for semantic memory.
Semantic memory refers to general world knowledge (facts, ideas, meaning and concepts) that can be articulated and is independent of personal experience.
This includes world knowledge, object knowledge, language knowledge, and conceptual priming. Semantic memory is distinct from episodic memory, which is our memory of experiences and specific events that occur during our lives, from which we can recreate at any given point.
For instance, semantic memory might contain information about what a cat is, whereas episodic memory might contain a specific memory of petting a particular cat. We can learn about new concepts by applying our knowledge learned from things in the past.
Other examples of semantic memory include types of food, capital cities of a geographic region, or the lexicon of a common language, such as a person’s vocabulary.
Autobiographical memory is a memory system consisting of episodes recollected from an individual’s life, based on a combination of episodic (personal experiences and specific objects, people and events experienced at particular time and place) and semantic (general knowledge and facts about the world) memory.
Spatial memory is the part of memory responsible for recording information about one’s environment and its spatial orientation. For example, a person’s spatial memory is required in order to navigate around a familiar city, just as a rat’s spatial memory is needed to learn the location of food at the end of a maze.
It is often argued that in both humans and animals, spatial memories are summarized as a cognitive map. Spatial memory has representations within working, short-term and long-term memory. Research indicates that there are specific areas of the brain associated with spatial memory. Many methods are used for measuring spatial memory in children, adults, and animals.
Although many psychologists believe that the entire brain is involved with memory, the hippocampus and surrounding structures appear to be most important in declarative memory specifically. The ability to retain and recall episodic memories is highly dependent on the hippocampus, whereas the formation of new declarative memories relies on both the hippocampus and parahippocampus Other studies have found that the parahippocampal cortices were related to superior Recognition Memory.
The Three Stage Model was developed by Eichenbaum, et. Al (2001), and proposes that the hippocampus does three things with episodic memory:
- Mediates the recording of episodic memories
- Identifies common features between episodes
- Links these common episodes in a memory space.
To support this model, a version of Piaget’s Transitive Inference Task was used to show that the hippocampus is in fact used as the memory space.
When experiencing an event for the first time, a link is formed in the hippocampus allowing us to recall that event in the future. Separate links are also made for features related to that event.
For example, when you meet someone new, a unique link is created for them. More links are then connected to that person’s link so you can remember what colour their shirt was, what the weather was like when you met them, etc. Specific episodes are made easier to remember and recall by repeatedly exposing oneself to them (which strengthens the links in the memory space) allowing for faster retrieval when remembering.
Hippocampal cells (neurons) are activated depending on what information one is exposed to at that moment. Some cells are specific to spatial information, certain stimuli (smells, etc.), or behaviours as has been shown in a Radial Maze Task. It is therefore the hippocampus that allows us to recognize certain situations, environments, etc. as being either distinct or similar to others. However, the Three Stage Model does not incorporate the importance of other cortical structures in memory.
The anatomy of the hippocampus is largely conserved across mammals, and the role of these areas in declarative memory are conserved across species as well. The organization and neural pathways of the hippocampus are very similar in humans and other mammal species. In humans and other mammals, a cross-section of the hippocampus shows the dentate gyrus as well as the dense cell layers of the CA fields. The intrinsic connectivity of these areas are also conserved.
Results from an experiment by Davachi, Mitchell, and Wagner (2003) and numerous subsequent studies (Davachi, 2006) show that activation in the hippocampus during encoding is related to a subject’s ability to recall prior events or later relational memories. These tests did not differentiate between individual test items later seen and those forgotten.
The lateral Prefrontal cortex (PFC) is essential for remembering contextual details of an experience rather than for memory formation. The PFC is also more involved with episodic memory than semantic memory, although it does play a small role in semantics.
Using PET studies and word stimuli, Endel Tulving found that remembering is an automatic process. It is also well documented that a hemispheric asymmetry occurs in the PFC: When encoding memories, the Left Dorsolateral PFC (LPFC) is activated, and when retrieving memories, activation is seen in the Right Dorsolateral PFC (RPFC).
Studies have also shown that the PFC is extremely involved with autonoetic consciousness (See Tulving’s theory). This is responsible for humans’ recollective experiences and ‘mental time travelling’ abilities (characteristics of episodic memory).
The amygdala is believed to be involved in the encoding and retrieval of emotionally charged memories. Much of the evidence for this has come from research on a phenomenon known as flashbulb memories.
These are instances in which memories of powerful emotional events are more highly detailed and enduring than regular memories (e.g. September 11 attacks, assassination of JFK). These memories have been linked to increased activation in the amygdala. Recent studies of patients with damage to the amygdala suggest that it is involved in memory for general knowledge, and not for specific information.
Lesion studies are commonly used in cognitive neuroscience research. Lesions can occur naturally through trauma or disease, or they can be surgically induced by researchers. In the study of declarative memory, the hippocampus and the amygdala are two structures frequently examined using this technique.
Hippocampal Lesion Studies
The Morris water navigation task tests spatial learning in rats. In this test rats learn to escape from a pool by swimming toward a platform submerged just below the surface of the water.
Visual cues that surround the pool (e.g. a chair or window) help the rat to locate the platform on subsequent trials. The rats’ use of specific events, cues and places are all forms of declarative memory.
Two groups of rats are observed: a control group with no lesions and an experimental group with hippocampal lesions. In this task created by Morris, et al., rats are placed in the pool at the same position for 12 trials. Each trial is timed and the path taken by the rats is recorded. Rats with hippocampal lesions successfully learn to find the platform.
If the starting point is moved, the rats with hippocampal lesions typically fail to locate the platform. The control rats, however, are able to find the platform using the cues acquired during the learning trials. This demonstrates the involvement of the hippocampus in declarative memory.
The Odor-odor Recognition Task, devised by Bunsey and Eichenbaum, involves a social encounter between two rats (a “subject” and a “demonstrator”). The demonstrator, after eating a specific type of food, interacts with the subject rat, who then smells the food odor on the other’s breath. The experimenters then present the subject rat with a decision between two food options; the food previously eaten by the demonstrator, and a novel food.
The researchers found that when there was no time delay, both control rats and rats with lesions chose the familiar food. After 24 hours, however, the rats with hippocampal lesions were just as likely to eat both types of food, while control rats chose the familiar food.
This can be attributed to the inability to form episodic memories due to lesions in the hippocampus. The effects of this study can be observed in humans with amnesia, indicating the role of the hippocampus in developing episodic memories that can be generalized to similar situations.
Amygdala Lesion Studies
Adolph, Cahill and Schul completed a study showing that emotional arousal facilitates the encoding of material into long term declarative memory. They selected two subjects with bilateral damage to the amygdala, as well as six control subjects and six subjects with brain damage.
All subjects were shown a series of twelve slides accompanied by a narrative.
The slides varied in the degree to which they evoked emotion – slides 1 through 4 and slides 9 through 12 contain non-emotional content. Slides 5 through 8 contain emotional material, and the seventh slide contained the most emotionally arousing image and description (a picture of surgically repaired legs of a car crash victim).
The emotionally arousing slide (slide 7) was remembered no better by the bilateral damage participants than any of the other slides. All other participants notably remembered the seventh slide the best and in most detail out of all the other slides. This shows that the amygdala is necessary to facilitate encoding of declarative knowledge regarding emotionally arousing stimuli, but is not required for encoding knowledge of emotionally neutral stimuli.
Factors That Affect Declarative Memory
Stress may have an effect on the recall of declarative memories. Lupien, et al. completed a study that had 3 phases for participants to take part in. Phase 1 involved memorizing a series of words, phase 2 entailed either a stressful (public speaking) or non-stressful situation (an attention task), and phase 3 required participants to recall the words they learned in phase 1.
There were signs of decreased declarative memory performance in the participants that had to complete the stressful situation after learning the words. Recall performance after the stressful situation was found to be worse overall than after the non-stressful situation, where performance differed based on whether the participant responded to the stressful situation with an increase in measured levels of salivary cortisol.
Posttraumatic stress disorder (PTSD) emerges after exposure to a traumatic event eliciting fear, horror or helplessness that involves bodily injury, the threat of injury, or death to one’s self or another person. The chronic stress in PTSD contributes to an observed decrease in hippocampal volume and declarative memory deficits.
Neurochemical Factors Of Stress On The Brain
Cortisol is the primary glucocorticoid in the human body. In the brain, it modulates the ability of the hippocampus and prefrontal cortex to process memories.
Although the exact molecular mechanism of how glucocorticoids influence memory formation is unknown, the presence of glucocorticoid receptors in the hippocampus and prefrontal cortex tell us these structures are some of its many targets. It has been demonstrated that cortisone, a glucocorticoid, impaired blood flow in the right parahippocampal gyrus, left visual cortex and cerebellum.
A study by Damoiseaux et al. (2007) evaluated the effects of glucocorticoids on hippocampal and prefrontal cortex activation during declarative memory retrieval. They found that administration of hydrocortisone (name given to cortisol when it is used as a medication) to participants one hour before retrieval of information impairs free recall of words, yet when administered before or after learning they had no effect on recall.
They also found that hydrocortisone decreases brain activity in the above-mentioned areas during declarative memory retrieval. Therefore, naturally occurring elevations of cortisol during periods of stress lead to impairment of declarative memory.
It is important to note that this study involved only male subjects, which may be significant as sex steroid hormones may have different effects in response to cortisol administration.
Men and women also respond to emotional stimuli differently and this may affect cortisol levels. This was also the first Functional magnetic resonance imaging (fMRI) study done utilising glucocorticoids, therefore more research is necessary to further substantiate these findings.
Donna Rose Addis, Morgan Barense, Audrey Duarte
The Wiley Handbook on The Cognitive Neuroscience of Memory
Wiley-Blackwell; (2015) ISBN-13: 978-1118332597